This invention relates to a record carrier, and particularly, though not exclusively, to a credit card or the like having a secure data storage means. It also relates to a method of labelling an article of value.
A known data carrier is described in GB-A-1,331,604, in which a credit card is provided with a secure data storage means comprising a layer having spaced regions in which anisotropic magnetic particles are dispersed and fixedly aligned in a binder along a pre-set direction. In remaining regions of the layer the particles are not so aligned or are aligned along a substantially different pre-set direction. This arrangement is commonly described as a magnetic xe2x80x9cwatermarkxe2x80x9d or as forming a xe2x80x9cpermanent magnetic structurexe2x80x9d since unlike conventional magnetic recordings the pattern of remanent magnetisation revealed by uniaxially magnetising or xe2x80x9cdevelopingxe2x80x9d the document can be restored by re-magnetisation even after erasure (by, for example, the application of an a/c erase field). A magnetic xe2x80x9cwatermarkxe2x80x9d is particularly well suited to recording data in digital form since each alignment direction may be assigned a different significance i.e. a binary xe2x80x9cONExe2x80x9d or a binary xe2x80x9cZEROxe2x80x9d. Such a data storage means is fairly secure because the patterns comprising the data cannot be erased because the structure is permanently aligned in a given direction.
Due to the xe2x80x9cbuilt-inxe2x80x9d nature of the recording, a data storage means may conveniently be prepared as a single xe2x80x9cwatermarkedxe2x80x9d tape comprising a sequence of binary coded numbers which provide a security feature when cut from the tape and applied to a support such as a document or card. Each of these identification numbers is different and is separated from the others by a marker or xe2x80x9csentinelxe2x80x9d comprising a unique sequence of binary digits which itself never appears in the sequence. For this reason the xe2x80x9csentinelxe2x80x9d can always be identified so that by reference to its position on the record carrier the digits chosen by the issuer of the record carrier as an identification number to be characteristic of the record carrier or to represent the security data (a particular set of characters appearing on the document, for example) can always be recovered.
However, due to tolerancing problems during manufacture of the tape and problems of registration between the tape and the support it is not possible without recourse to the use of expensive equipment, to ensure that a xe2x80x9csentinelxe2x80x9d or xe2x80x9csentinelsxe2x80x9d always appear in the same position on the document.
In practice, when one xe2x80x9csentinelxe2x80x9d appears at each end of the document the digits between the xe2x80x9csentinelsxe2x80x9d are chosen to represent the security data. It sometimes occurs, however, that only one xe2x80x9csentinelxe2x80x9d appears on the document so that depending upon the exact location an appropriate number of digits, selected from both sides of the xe2x80x9csentinelxe2x80x9d, are chosen to represent the security data identifying that document. Thus for each position of the first detected xe2x80x9csentinelxe2x80x9d there is a different binary digit selection rule for choosing the bits constituting the security data. The sequence of bits representing the security data may then be used as verification information, or may be used to derive verification information, which for example, the user may have to supply before the card is validated. Such verification information is preferably unique and characteristic of the given record carrier.
This technique, however, has a weakness for the case when the first xe2x80x9csentinelxe2x80x9d on the card is close to an edge. In such a case it is possible for the card reader to fail to detect the first xe2x80x9csentinelxe2x80x9d and instead pick up the second xe2x80x9csentinelxe2x80x9d on the card. Thus there is always the possibility of an ambiguity. For instance suppose that a card is made in a factory, and a piece of magnetic tape is attached to the card, the tape having the number 300 together with parts of the numbers 299 and 301. The card is read in the factory and the identification number 300 is obtained. Without knowing how the xe2x80x9csentinelsxe2x80x9d lie with respect to the edge of the card, it is possible for card readers in the field to recognise the identification number of the card as 299, 300 or 301. This is clearly undesirable for cards for use in financial transactions, as it might be possible to confuse two cards having adjoining lengths of xe2x80x9cwatermarkedxe2x80x9d tape attached to them. It is also undesirable for cards used for claiming benefit, or access control, or identification, or having a stored value such as prepaid fare cards.
In order to solve this problem, xe2x80x9cWATERMARKxe2x80x9d tape made by THORN Secure Science Limited, Swindon, England, is often applied to cards in a registered fashion such that the xe2x80x9csentinelsxe2x80x9d always occur at the same point on a card. Increased production costs are associated with such registered cards.
Another method of making security documents is disclosed in GB-A-2 021 835. In this method the position of the sentinel is erasably recorded on a different portion of the document, thus eliminating ambiguity.
According to a first aspect of the invention, there is provided a record carrier. This can provide the advantage of identifying the record carrier uniquely without having to resort to expensive registration schemes to ensure that the markers are always at exactly the same position on the record carrier.
The invention is particularly advantageous for credit cards and the like provided with permanently structured magnetic (xe2x80x9cwatermarkxe2x80x9d) tape, as it allows the tape to be placed freely on the carrier without the risk of introducing ambiguity.
According to a second aspect of the invention there is provided a method of labelling an article of value.